The paradox of being polyploid – experimental evolution using Chlamydomonas

Type:

Master Thesis subject (30 ECTS)

programme:

EMBC+

Oceans and Lakes

Polyploids, organisms that underwent a whole genome duplication (WGD), possess one or more extra copy of their genome, which is free to evolve new functionality. Many WGDs are accordingly found in very successful eukaryotic lineages characterized by particular biological innovations and/or diversifications. However, the possession of two complete genomes is generally not well tolerated by cells leading to harmful effects on the overall fitness and fertility? of the organism. This results in a paradox between the observed immediate negative effects of WGD, and their attributed long-term positive effects. Previous work demonstrated that many plant polyploids became established during the last mass extinction event. Present-day polyploids are also more abundant in stressful environments such as the Arctic. We therefore hypothesize that stress can enhance polyploid establishment because their plastic phenotypes and genotypes can enable rapid adaptive changes that allow survival. To test this hypothesis, we propose an evolutionary experiment based on unicellular green algae (the model system Chlamydomonas), of which we make polyploid strains and subject them to a wide variety of stressors to quantify their survival. Second, we will perform competition assays to compare the fitness of polyploids with the non-polyploid ancestors.
Practical work will involve: Setting up an evolutionary experiment; Asexually batch culturing of established diploid and haploid lines for over 300 generations in various environmental stress regimes (to allow the cells to evolve). Evaluation of the end populations for adaptation rate, fitness and therefore, evolutionary success of haploid and diploid lines. Determination of the occurrence of new diploid lines arising spontaneously under stress. Isolation of genomic DNA in every 50 generations for later sequencing and genotyping experiments (NGS-Illumina) which will allow us to investigate genome evolution following polyploidization, leading the success of a species.